FRIDAY, April 12 (HealthDay News) -- The more sleepteens get, the less likely they are to be overweight, a new study finds.

Researchers followed more than 1,000 Philadelphia-area teenagers from their freshman through senior years of high school. Every six months, the teens reported their sleep patterns.

The results showed that each additional hour of sleep per night was associated with reduced body-mass index (BMI), a measure of body fat based on height and weight. The reduction was greatest in those with higher BMIs, meaning they were more overweight.

Based on their findings, the researchers from the University of Pennsylvania Perelman School of Medicine suggested that increasing sleep from eight to 10 hours a night at age 18 could lead to a 4 percent reduction in the number of overweight or obese American teens. That would translate to about 500,000 fewer overweight teens.

The study was published online April 8 in the journal Pediatrics.

"Educating adolescents on the benefits of sleep and informing them of sleep hygiene practices has shown to have little impact on adolescent sleep duration," lead author Jonathan Mitchell, a postdoctoral fellow in the Center for Clinical Epidemiology and Biostatistics, said in a Penn Medicine news release.

"One possible solution could be for high schools to delay the start of the school day," Mitchell said. "Previous research has shown that delaying the start of the school day even by 30 minutes results in a 45-minute per day increase in sleep. Since our study shows increasing sleep by an hour or more could lead to a lower BMI, delaying the start of the school day could help reduce obesity in adolescents."

Although the research suggested an association between more sleep and a lower BMI, it didn't establish a cause-and-effect relationship.